This invention relates to a thermal air flow meter. More particularly, the present invention relates to a thermal air flow meter which is suitable for measuring the quantity of air taken in by an internal-combustion engine of an automobile.
In an internal-combustion engine of an automobile equipped with a fuel injection system, the operational state of the internal-combustion engine is detected by signals from various sensors, and the quantity of fuel injected by the fuel injection system, etc., is controlled. A thermal air flow meter is one of these sensors, it is used for measuring the quantity of air taken in by the engine. Thermal air flow meters of various types are known, but thermal air flow meters using temperature-dependent, heat-sensitive resistors have gained a wide application because they provide a high measurement accuracy. As disclosed in U.S. Pat. No. 4,264,961, for example, an advantageous thermal air flow meter has a construction in which part of the air flowing through an intake pipe is introduced into a by-pass pipe past a heat-sensitive resistor disposed in this by-pass pipe. This is advantageous because such a flow meter undergoes hardly any mechanical damage due to backfires that can occur when the engine is not running properly.
The heat-sensitive resistor has the following construction. Platinum wire of 20 .mu.m diameter is wound around an alumina bobbin which is 0.5 mm in diameter and 2 mm long to form the heat-sensitive resistor for the thermal air flow meter. Lead wires consisting principally of platinum are bonded to both ends of the bobbin by adhesive, and the lead wires are spot-welded to supports. The thermal air flow meter using this heat-sensitive resistor has the problem that when the flow rate changes suddenly, particularly when the flow rate increases suddenly, the response of the flow meter drops.
Flow meters of this construction are used mainly in multi-point fuel injection systems. Recently, engines with single point fuel injection systems have been examined. It has been found that when a flow meter of the construction described above is used in a single point fuel injection system, its response during acceleration is low, and this must be improved. In a single point fuel injection system, a single fuel injection valve is provided at the point at which the intake pipes of the engine join, and hence the distances from the fuel injection position to the cylinder inlets are longer than those of a multi-point fuel injection system. This means that the time taken for the fuel to arrive at each cylinder is longer. The distance from the fuel injection position to each cylinder varies from cylinder to cylinder. This means that if the shape of the intake pipes changes because of changes to the engine, delicate matching must be carried out. Accordingly, the detection accuracy must be improved by use of a very accurate flow meter, particularly at high speeds, so as to accurately follow pulsations in the flow of intake air to the engine.
As a result of examining the response of thermal air flow meters, it has been found that when the flow rate is changed in steps, a time constant 1.tau. (the time at which 63% of full scale is reached) is rapid but 3.tau. (the time at which 95% of full scale is reached) is slow. 1.tau. is mainly determined by the response of the driving circuit of the flow meter, while 3.tau. is determined by the thermal characteristics of the heat-sensitive resistor and its supports.